1. Field of the Invention
The present invention relates to a flow quantity test system for a mass flow controller usable with a gas system in a semiconductor fabrication process, and more particularly to a flow quantity test system capable of testing precision in flow quantity measurement for a mass flow controller that has been assembled in the gas system.
2. Discussion of the Prior Art
In an apparatus for forming a membrane, an apparatus for dry etching and the like, utilized in semiconductor fabrication process, used are such a specific material gas as for instance silane, phosphine or the like, a corrosive gas such as a chroline gas or the like and a highly combustible gas such as a hydrogen gas or the like. For usage of these gases, a flow quantity of each gas must be managed in a significantly strict manner due to the following reasons.
The first reason, is the fact that the quantity of the gas flow affects directly the quality of the process. That is, precision in the gas flow quantity enormously influences the membrane quality in a process for forming a membrane and also the quality of the circuit in the etching process.
Another reason, is the fact that most of these kind of gases have injurious or explosive properties. Thus, these gases after usage may not be permitted to directly exhaust into the atmosphere. As a result, it is required to prepare means for eliminating injury according to the kind of gases. The means for eliminating injury is restrained usually in its processing ability, resulting often in flowing out of an injurious gas into the environment and damaging of the means for eliminating injury when the flow quantity becomes more than a permissible value.
Furthermore, still another reason is the fact that these gases, especially high purity and dust-free gases usable in semiconductor fabrication processes are expensive and that storage in large amounts of the gases is difficult because of time limit in use caused by natural deterioration depending on the kind of the gases.
Meanwhile, the actual flow quantity of these gases requested by process equipment is little or at most 500 sccm. For this reason, a conventional mass flow controller is disposed within a gas line to flow each gas with an optimum flow quantity for every kind of gas. A setting flow quantity for the mass flow controller may be altered in relation to an applied voltage thereto to meet with changes of the process recipe.
For the purpose of controlling a small flow quantity, the mass flow controller for gas process of this kind is provided with a capillary tube that acts to monitor flow quantity according to its operation. Meanwhile, solid bodies may be separated in the capillary tube from a material gas for forming a membrane among gases flowing through the mass flow controller due to the property of the mass flow controller, often causing change in a flow quantity capacity of the capillary tube. If this change is caused, the relationship between an applied voltage and the actual flow quantity of the mass flow controller is naturally changed. In other words, because the actual flow quantity changes without any change in setting of the applied voltage, the process may be prevented from being stable. In case such a change actually occur, the setting of the applied voltage must be corrected in order to flow a proper gas flow quantity. In this instance, it is required to measure the actual flow quantity of the mass flow controller.
Since monitoring of the flow quantity becomes impossible due to logging of the capillary tube when the separated solid bodies are further accumulated, it is difficult to cope with the above-mentioned prevention of the stability of the process. Such continuous usage of the mass flow controller prior to the accumulation induces to apply to the process instrument particles that are undesirable in fabrication of semiconductors, causing undesirable results. In this case, the mass flow controller must therefore be replaced with a new one. Individual differences in relationship between the applied voltage and actual flow quantity of the mass flow controller may not be neglected even if the mass flow controller is the same type and a fastening degree of a joint with the conduit system affects the actual flow quantity. It is therefore required to measure the actual flow quantity of the newly replaced mass flow controller.
However, it is hardly carried out till now to measure the actual flow quantity of the mass flow controller because it is difficult to measure the actual flow quantity of the mass flow controller that has been assembled in the conduit system. Thus, an applied voltage is provisionally set by intuition and experience of a worker for measurement of the actual flow quantity. A process is carried out to determine the quality of the provisional value according to the quality of the process and these procedures are repeated to determine an optimum setting value.
As a result, it takes time to determine the provisional value. This causes lowering the actual availability of the process apparatus and has costs including the consumption of various gases and test wafers in the process.